Your question re:
8. Uccasayana-mahasayana veramani sikkhapadam samadiyami
I undertake the precept to refrain from lying on a high or luxurious sleeping place.
How high it means?
Does this include refraining of lying on a mattress that put on the floor?
What is the correct practice for this precept? Should the observer lay only on naked floor without pillow or any cushions? (I had tried it and wow my tailbone hurt , but it's a good experience though).
Is sleeping on plaited mat permissible?
What about the pillow or any cushions? Is it permissible too?
this this is how I see it.
The objective of this precept is three folds.
a) Elimination of attachment to sensual pleasure. (comfortable beds etc)
b) Keep your mind alert (When you are too comfortable you may go to sleep and not easy to contemplate on stress – Dukkaha)
c) Most important part is to eliminate pride (Mana - tenth fetter) Can you imagine a king, queen or president of a state sitting on the floor. Only a humble person can do it.
If you keep above in your mind you will find the answers to your questions.